In order to give some idea of the dimensions used, it is noted that a unit in a power plant with an output of 200 kW, which can be considered nowadays as a facility with a small output, can have the following parameters for dry-cooling:
______________________________________ heat to be extracted 300 MW cooling water flow 7 m.sup.3 /s height of the tower 120 m lower diameter 110 m ribbed surface area of its air radiators 600 000 m.sup.2 air-cooling mass 600 tons ______________________________________
With such dimensions, natural-draft chimney cooling towers are considered as economical. Draft and air flow caused thereby are formed almost immediately, as soon as warm water arrives at the air radiator.
The prior art provides many proposals for dry-operated cooling towers with both natural draft and forced circulation of the cooling air, for example U.S. Pat. No. 4,747,980 Bakay et al. and German Patent Application No. 28 36 053.
In the dry-operated cooling towers movable or adjustable jalousies are frequently used to control the air flow and regulate the output of the tower.
Previously, jalousies for the control of air flow have been used both with cooling towers having ventilators and with chimney cooling towers. Jalousies are effective for regulating heat output in two different types of cooling tower most widely used in practice. One type of cooling tower having what is known as a "Heller-arrangement" is characterized in that air radiators are installed along the lower diameter of the tower, next to each other with water flowing vertically in the pipes. Cooling air is led horizontally into the tower and exits up the chimney. Jalousies are installed before the coolers. This technique has the advantage that the jalousies offer particularly in their closed position protection for the air radiators against both damage and contamination.
Such a construction is suggested in Bakay et a which states that its teaching is applicable to natural draft cooling towers although the specific teaching focuses on forced-draft systems.
A second type of cooling tower uses air radiators which are arranged horizontally inside the tower. Air radiators can be installed either radially or parallel with each other. In this case also, the jalousies serving for the regulation of the air flow are arranged on the inflow side of the air, i.e. they are arranged under the air radiators.
Both these techniques are effective for the control of the mass of air streaming through the air radiators and, accordingly, for regulating the output of the air radiators. They are also useful when disconnecting the air radiators to take them out of operation.
A well-known problem associated with the operation of dry cooling towers in cold weather and, in particular, with start-up in cold weather, is frosting or freeze up. Various solutions have been proposed which pre-heat the air radiators prior to filling them with water.
One solution for pre-heating air radiators became known as the "Heller-towers" and employs the feature that between the vertically arranged coolers and the regulating jalousies there are arranged smaller dimensioned ventilators blowing-in warm air. The air is allowed to stream through the air radiators and heats them gradually. The ventilators can include an air radiator also heated with water, however, this is far smaller than the cooling radiators. Accordingly, neither start-up nor filling the ventilators is considered to present any danger of freeze up. However, this type of pre-heating has the drawback of requiring a considerable expenditure on heating and of generating an intense air flow, as the warm air leaves the chimney and is lost.
Some problems which may arise are: When the cooling tower is started-up and the heat received from the medium to be cooled is not sufficient, the water system can become unduly cooled and may ice up.
If a previously disconnected group of air radiators is brought into operation frosting may arise.
Adjustable, that is, movable jalousies used in dry-operated cooling towers, may reduce the air flow within the tower to such an extent that an adequate warm-air draft cannot be created. The warm air is unable to fill out the whole cross-section of the relevant radiator, while local motion of air tending to move upwards may be equalized or offset by heavy cold air entering on the top of the tower.
Such air radiators usually have a plurality of parallel-connected water pipe are arranged. So e.g for example, referring to the cooling tower mentioned above, water may flow through as many as 30,000 pipes with a diameter of 17 mm, and a length of about 30 m. As mentioned above, in the [single] individual pipes or on the surface of the air radiator, freezing may occur, resulting in damage, or blockages in the pipes and in the air radiators. It is clear that 600 tons of cold metal mass, according to our example, is readily able to freeze water during filling and, to seal the pipes by icing. Frost may also occur during discharge, in such a manner that water is discharged too slowly and the remaining water is frozen.
In order to achieve a frost-free filling and discharge, it is known to pre-heat the cold air radiators and keep them warm, by stopping air-flow through the air radiators during filling and discharge.
A common characteristic of known pre-heating equipment is the blowing of warm air into the outer side of the air radiator. The warm air, after having passed through the air radiator, flows directly to the chimney of the cooling tower and is lost. With the dimensions and outputs of the example above, such lost heat is substantial and expensive.